In recent years, along with the rapid expansion of small-sized electronic devices such as cellular phones and laptop computers, a demand for a lithium-ion secondary battery as a chargeable and dischargeable power source has been rapidly increased. A lithium-cobalt oxide (hereinafter, sometimes also referred to as cobalt-based) has been widely used as a positive-electrode active substance contributing to the charging and discharging in a positive electrode of a lithium-ion secondary battery. However, capacity of the cobalt-based positive electrode has improved to the extent of theoretical capacity through the optimization of battery design, and higher capacity is becoming difficult to achieve.
Accordingly, lithium-nickel composite oxide particles using a lithium-nickel oxide that has the theoretical capacity higher than that of the conventional cobalt-based one have been developed. However, the pure lithium-nickel oxide has a problem in terms of safety, cycle characteristics, and the like because of the high reactivity with water, carbon dioxide, or the like, and is difficult to be used as a practical battery. Therefore, a lithium-nickel composite oxide to which a transition metal element such as cobalt, manganese, and iron, or aluminum is added has been developed as an improvement measure for the problem described above.
In the lithium-nickel composite oxide, there are composite oxide particles expressed by a transition metal composition of Ni0.33Co0.33Mn0.33, a so-called ternary composite oxide (hereinafter, sometimes referred to as ternary), which is made by adding nickel, manganese, and cobalt in an equimolar amount, respectively, and lithium-nickel composite oxide particles with a nickel content exceeding 0.65 mol, a so-called nickel-based composite oxide (hereinafter, sometimes referred to as nickel-based). From the viewpoint of capacity, a nickel-based with a large nickel content has a great advantage as compared to a ternary.
However, the nickel-based is characterized by being more sensitive depending on the environment as compared to a cobalt-based or a ternary, because of the high reactivity with water, carbon dioxide, and the like, and absorbing moisture and carbon dioxide (CO2) in the air more easily. It has been reported that the moisture and carbon dioxide are deposited on particle surfaces as impurities such as lithium hydroxide (LiOH), and lithium carbonate (Li2CO3), respectively, and have an adverse effect on the production process of a positive electrode or battery performance.
By the way, the production process of a positive electrode passes through a process in which a positive electrode mixture slurry obtained by mixing lithium-nickel composite oxide particles, a conductive auxiliary, a binder, an organic solvent, and the like is applied onto a collector made of aluminum or the like, and dried. In general, in the production process of a positive electrode mixture slurry, lithium hydroxide causes the slurry viscosity to increase rapidly by reacting with a binder, and may cause gelation of the slurry. These phenomena cause faults and defects, and a decrease of production yield of a positive electrode, and may cause a variation in quality of the products. Further, during charging and discharging, these impurities react with an electrolytic solution and sometimes generate gas, and may cause a problem in the stability of the battery.
Accordingly, in a case where a nickel-based is used as a positive-electrode active substance, in order to prevent the generation of impurities such as the above-described lithium hydroxide (LiOH), the production process of a positive electrode is required to be performed in a dry (low humidity) environment in a decarbonated atmosphere. Therefore, there is a problem that in spite of having high theoretical capacity and showing great promise as a material of a lithium-ion secondary battery, the nickel-based requires high cost for the introduction of a facility and high running costs for the facility in order to maintain the production environment, and which becomes a barrier to it becoming widespread.
In order to solve the problem described above, a method of coating surfaces of lithium-nickel composite oxide particles by using a coating agent has been proposed. Such a coating agent is roughly classified as an inorganic coating agent and an organic coating agent. As the inorganic coating agent, a material such as titanium oxide, aluminum oxide, aluminum phosphate, cobalt phosphate, fumed silica, and lithium fluoride have been proposed, and as the organic coating agent, a material such as carboxymethyl cellulose, and a fluorine-containing polymer have been proposed.
For example, in Patent Document 1, a method of forming a lithium fluoride (LiF) or fluorine-containing polymer layer on surfaces of lithium-nickel composite oxide particles has been proposed, and in Patent Document 2, a method of forming a fluorine-containing polymer layer onto lithium-nickel composite oxide particles, and further adding a Lewis acid compound to neutralize impurities has been proposed. In any processing, the surfaces of lithium-nickel composite oxide particles are modified so as to have the hydrophobic property with a coated layer containing a fluorine-based material, and adsorption of moisture is suppressed, and the deposition of impurities such as lithium hydroxide (LiOH) can be suppressed.
However, the coated layer containing the above-described fluorine-based material, which is used in these coating methods, is merely attached onto lithium-nickel composite oxide particles only by electrostatic attraction. Accordingly, the coated layer is redissolved in N-methyl-2-pyrrolidone (NMP), which is used as a solvent in the slurry production process, therefore, the coated layer is easily detached from the lithium-nickel composite oxide particles. As a result, the positive electrode is required to be stored in a dry (low humidity) environment in a decarbonated atmosphere, and not only cannot the faults and defects and the decrease of production yield, which are problems in the nickel-based, be suppressed, but also the problem with the stability of a battery substantially due to the generation of impurities cannot be thoroughly solved.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2013-179063
Patent Document 2: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2011-511402